Ophthalmologists routinely treat patients that suffer from impaired vision caused by a condition known as traction maculopathy. This condition, which is the result of tractional forces that distort or wrinkle the center of the retina (macula), may affect as many as 2.5 million patients. Moreover, traction maculopathies, in the form of vitreomacular traction syndrome, cellophane maculopathy, epimacular proliferation, and macular hole disease, are said to occur in 6.4% of the U.S. population over 50 years of age.
For some time, ophthalmologists have recognized the effectiveness of treating traction maculopathy with surgical intervention. In the surgical procedure, the eye surgeon performs a series of surgical steps that eliminate the tractional forces from the macula. Recently, with the development of advanced high-powered surgical microscopes, and other modern surgical techniques, such as the Indocyanine Green (ICG) staining technique of the internal limiting membrane, surgeons have been able to execute surgical manipulations on tissues of the macula that were previously difficult to visualize.
The use of modern surgical techniques, however, creates additional concerns that a surgeon must keep in mind while completing a surgical procedure to reduce tractional forces on the macula. When macular surgery is performed, the surgeon must use an endoscopic light probe that produces high-powered illumination. Because there is no absorbing barrier between this light source and the retina, this illumination can damage the retina. F. Kuhn, R. E. Morris, & M. P. Massey, “Photic Retinal Injury from Endoillumination during Victrectomy,” American Journal of Ophthalmology, 111:42-46 (January, 1991). Such damage to the retinal tissues is referred to as phototoxicity or photic retinopathy.
The damaging effect of medical instrument illumination has been recognized for three decades. In 1973, Tso described photic retinopathy lesions that were intentionally produced in the eyes of a monkey. M. Tso, Investigative Ophthalmology, 12: 17-34 (1973). Later, McDonald and Irvine described a group of patients after cataract extraction that had similar lesions. H. R. McDonald & A. R. Irvine, Ophthalmology, 90: 945-951 (1983). Such lesions have also been intentionally demonstrated by experiments that exposed human eyes, prior to the removal of the eye for an unrelated malignant tumor, to an operating microscope light for sixty minutes. W. R. Green W R & D. M. Robertson, American Journal of Ophthalmology, 112: 520-527 (1991).
The prevalence rate of retinal phototoxicity has been estimated to range from 3% to 7.4% after surgery for cataract extraction using the illuminated microscope. S. G. Khwarg, F. A. Linstone, S. A. Daniels, et al., American Journal of Ophthalmology, 103: 255-263 (1987); J. E. Gomolin & R. K. Koehekoop, Canadian Journal of Ophthalmology, 28: 2121-224 (1993). Even if a characteristic retinal burn is not present, it has been postulated that subtle, chronic cystoid macular edema may result. At its worst, phototoxicity can produce permanent legal blindness in the affected eye.
Normal physiological protection of the human retina against phototoxicity is partly provided by the ocular media, which filters or absorbs the most damaging ultraviolet rays. The pupil is also capable of constricting in response to bright light, thus reducing light transmission to the retina by more than 80%. R. E. Records & J. L. Brown, Adaptation in Duane's Foundations of Clinical Ophthalmology, vol.2, ch. 16, Tasmas and Jaeger Editors (1991). Finally, if light is too intense, the eyelids will close, or the brain may turn the gaze of the eyes away from the offending light source. Unfortunately, during macular surgery, all of these protecting mechanisms are bypassed.
For over twenty years, various efforts have been made to understand and prevent phototoxicity during eye surgery. The filtering of shorter wavelength light lessens but does not eliminate retinal damage. R. H. Keates & P. R. Armstrong, Ophthalmic Surgery, 16: 40-41 (1985). Similarly, the filtering of infrared light greater than 700 nanometers is only partially helpful. M. A. Mainster, W. T. Ham, F. C. Dehori, Ophthalmology, 90: 927-932 (1983). Directing the endoscopic light probe away from the macula as much as possible, keeping the light probe as far from the macular surface as possible, and decreasing the endoscopic light intensity are the variables the surgeon can control.
Specifically, there is also the continued concern that the use of high intensity light may cause phototoxic injury to the foveal and macular tissues. Additionally, there remains a concern that the use of chemicals, such as Indocyanine Green (ICG) staining to identify intraocular structures may expose the foveal and macular tissues to injury from chemical toxicity.